Regenerative fuel cells, and the methods by which they are able to store and deliver electricity, have been known for many years. They are electrochemical apparatus for energy storage and power delivery. In the power delivery phase, electrochemically active species are supplied to electrodes, where they react electrochemically to produce electrochemical power. In a storage phase, electrical power is used to regenerate the electrochemically active species, which are stored.
Because the electrochemically active species can be stored separately from the electrode compartments and supplied when required, the generating capacity of this equipment can be quite large and scalable.
The electrochemical reactions take place on either side of an ion transport system (such as a membrane) with charge carriers either being transported or exchanged by the membrane.
The fundamental electrochemical process in these regenerative fuel cell (RFC) systems can be described by a simple redox equation where the action proceeds in one direction in the energy storage mode of the system and in the opposite direction during the power delivery mode of the system and the term “redox” defines reactions in which a reduction and a complementary oxidation occur together.
However the implementation of these systems in practical applications has encountered major limitations, despite what appears to be a simple electrochemical process. Practical problems have included the use of hazardous materials, poor efficiencies, system size, plugging and clogging of the flow of the electrolytes, crossover of the species, gas formation and, especially, the cost of materials and the cost of equipment. These have prevented RFCs from being employed widely in industry.
There is a wide range of potential applications for energy storage technologies. Most renewable energy technologies cannot easily adjust their power output to meet fluctuating demand and therefore energy storage is important in enabling low carbon/renewable energy sources to be implemented in practice. Energy storage technologies may also be used as a remote power source, to ensure constant power supply and quality and may also be used to reduce the cost of electricity by storing energy when electricity is cheap and distributing the stored energy at peak times.
One of the disadvantages faced by all regenerative fuel cells that have a metal redox couple is that hydrogen and/or oxygen co-evolution may occur when the metal ions are electrochemically reduced.
A liquid/gas fuel cell which employs hydrogen gas and liquid bromine electrolyte has been investigated by Livshits et al. (Electrochemistry Communications, 2006, vol. 8(8), 1358-62). Later, the hydrogen-bromine fuel cell was adapted into a RFC by EnStorage Flow Systems. Although this system has demonstrated a high discharge output power, there are a number of drawbacks to using this system including a low catalyst stability and bromine gas evolution during operation. More recently WO2011/089518 has proposed a hydrogen-bromine regenerative fuel cell and also mentions using a hydrogen-iron redox system. However, due to the low standard electrochemical potential of the iron II/III redox couple (0.77 V vs SHE), the average working voltage of such a hydrogen-iron system at discharge will be even lower (indeed it is the lowest among known regenerative fuel cells) which is a significant disadvantage for practical redox battery applications. Another liquid/gas RFC which has been investigated is the vanadium/air RFC (Hosseiny, S. S., et al, Electrochemistry Communications, 2011, vol. 13, 751-754); however this system has low efficiency, low power density and poor rechargeability.
It is important to realise that regenerative fuel cells are distinct from standard fuel cells. Standard fuel cells consume fuel and can normally only be run in a power delivery mode; they either cannot be run in an energy storage mode (in which power is stored) or, if they can, they can only do so in a highly inefficient way. Furthermore, reversing the electrochemical reaction in a fuel cell can cause permanent damage to the catalyst. Standard fuel cells are optimised for operating in the energy generating mode only while regenerative fuel cells are optimised for the combined power delivery mode and the energy storage mode. Thus only electrochemical reactions that are readily reversible can be used in a regenerative fuel cell, while in certain fuel cells (such as direct alcohol, or direct borohydride fuel cells, or hydrogen/oxygen fuel cells) the reactions need not be reversible and indeed they are usually not. Because of these considerations, regenerative fuel cells will normally use at least one different electrochemical reaction, as compared to standard fuel cells, although where a fuel cell clearly uses half cells that both use a readily reversible redox reaction, e.g. the hydrogen-I system disclosed in “Advancements in the Direct Hydrogen Redox Fuel”, Electrochemical and Solid-State Letters, 11 (2) B11-B15 (2008), such a system can be used both in fuel cells and regenerative fuel cells.
In addition the average operating voltage during discharge is important. A low voltage system will require either a higher number of cells in electrical series to increase the voltage, or the design of bespoke power converters to deal with low voltage-high current systems, which adds both complexity and cost to the system.
Therefore finding two redox couples for use in a regenerative fuel cell that are reversible, soluble at practical concentrations (about 1 M or above), have a suitable potential difference between the standard electrode potentials (E⊖/V) of the couples and overcome the problems in the art is a challenging task.
WO2013104664 (A1) overcomes the above problems by providing a hydrogen gas/dissolved metal ion regenerative fuel cell where the metal is selected from vanadium, cerium, manganese or their stable and electrochemically reversible aqueous complexes. Vanadium, cerium and manganese have relatively high electrochemical redox potentials:
Redox reactionStandard potential E0Ce4+ + e−   Ce3+1.72 VMn3+ + e−   Mn2+1.54 VVO2+ + 2H+ + e−   VO22+ + H2O0.99 V
In particularly preferred embodiments, WO2013104664 (A1) provides a hydrogen gas/dissolved vanadium ion regenerative fuel cell. The inventive regenerative fuel cells, and especially the hydrogen/vanadium ion system, at least partly overcome the problems of the all-vanadium RFCs (VRBs) currently in use in that costs are significantly reduced by halving the amount of expensive vanadium required. Furthermore, replacing large liquid electrolyte storage tanks with compressed gas storage vessels for hydrogen dramatically reduces the amount of space taken up by the regenerative fuel cell, which further reduces costs. Other advantages include an increase in output power of the system due to the lower overpotential of the hydrogen oxidation reaction. The preferred hydrogen/vanadium RFC provides a further advantage in that existing vanadium/vanadium RFC systems can be readily adapted to replace the vanadium anode with a hydrogen anode, thereby reducing the capital cost of installing the regenerative fuel cells of the present invention by preventing the need to install an entire system. This retro-fitting aspect therefore overcomes a substantial draw-back of using new systems for those who have already invested considerable capital in existing technology and is an important aspect of the present invention.
Despite the problems overcome by the RFCs disclosed in WO2013104664 A1, the issues of poor efficiencies, system size, plugging and clogging of the flow of the electrolytes, ion exchange membrane fouling and high capital cost, remain, preventing RFCs from being employed widely in industry.
For example, Mn is an abundant and cheap metal with high redox potential (E⊖−Mn(III)/Mn(II)=1.51 V) interesting for energy storage applications. However, the implementation of Mn-based electrolyte in regenerative fuel cells has been elusive to this date due to undesired precipitation reactions. During cell charging Mn(III) is produced and as shown below in Equation 2, this ion spontaneously undergoes disproportionation leading to precipitation of MnO2 with a consequent drop in cell capacity.2Mn3++2H2O→Mn2++MnO2+4H+  Equation 1
Higher metal solubility in the liquid phase permits higher cell energy density and reduces the frequency of plugging and clogging of the flow of the electrolytes and reduces ion exchange membrane fouling. In this context, the present invention concerns liquid electrolytes containing organic and/or inorganic additives in hydrogen (anode)/liquid electrolyte (cathode) regenerative fuel cells leading to an improved performance. This has a dramatic positive impact on the cost of the system, efficiency and energy density of the cell unreported to this date.
Regenerative fuel cells can generally be distinguished from fuel cells by their “plumbing”. A regenerative fuel cell has both conduits for supplying fuel to the electrodes for the power delivery phase, and also conduits for conducting the spent fuel to a store so that it can be regenerated. Often the fuel will be in the form of electrolyte that is exhausted following a power delivery phase and in this case the conduits will also be arranged to conduct exhausted (or spent) electrolyte to a store and supply it back to its half cell during an energy storage mode, e.g. by the use of appropriate pumps. In contrast, fuel cells are not set up to operate in energy storage mode to electrochemically replenish exhausted electrolyte. In the case of regenerative fuel cells having a half cell containing a gas electrode, a compressor is generally provided to compress gas generated during the energy storage mode to enable it to be collected in a compressed gas storage tank for future power delivery phases (although neither a compressor nor a storage tank is required where the electrode is an oxygen electrode since oxygen is freely available from the atmosphere). In contrast a fuel cell will generally not have such a compressor.